Youichi Kawasa

Liquid immersion lithography at 157 nm

We performed an initial evaluation of 157-nm immersion lithography. The 157-nm immersion fluid needs to have both a high refractive index and high transmittance at a wavelength of 157 nm. This paper focuses on the transparency of the fluid. We evaluated the transparency of straight-chain perfluoroalkane and perfluoroether using a semi-empirical molecular orbital method. We found that perfluoroether has lower absorption at 157 nm than perfluoroalkane, and increasing the amount of ether bonds in perfluoroether can further reduce the absorption. Moreover, we found that designing the molecular structure with ether bonds so that the number of successive CFx is balanced should further improve transparency. Although the commercial perfluoropolyether BARRIERTA® J25V contains a trifluoromethyl group in one of its side-chains, it satisfied the above conditions and achieved high transmittance of 1.0 cm-1 at 157 nm. The sensitivity characteristics of the XP2332C and F-SSQ resists were evaluated by dry and immersion exposure using BARRIERTA® J25V immersion fluid, and no noticeable changes were seen in the development contrast for either exposure condition for either of these two resists. To perform 157-nm immersion exposures, we constructed a Michelson interferometric exposure tool, which let us create an interference pattern with sufficient optical contrast. We obtained a resolution of 60-nm line-and-space pattern having a good rectangular shape by immersion exposure using this interferometric exposure tool, F-SSQ resist, and BARRIERTA® J25V immersion fluid without using a top-coat.

Evaluation of outgassing from a fluorinated resist for 157nm lithography

We have evaluated the outgassing products and the in situ transmittance of a contaminated CaF2 substrate for monocyclic fluoropolymers with four protecting groups: methoxymethyl (MOM), tert-butoxycarbonyl (t-BOC), menthoxymethyl, and 2-cyclohexylcyclohexyloxymethyl. We found little correlation between the total amount of outgassing from the polymer and the decreasing rate of the CaF2 substrate transmittance caused by outgassing adhesion. Although the MOM protecting group generated the largest amount of outgassing products, the most substantial decrease in the transmittance was observed for the t-BOC protecting group. Therefore, the absorption coefficient of the outgassing-contaminated CaF2 substrate appears to be more sensitive to the type of protecting group, especially the t-BOC protecting group including a t-butyl unit. We conclude that in terms of material design of the fluoropolymer resist for 157nm lithography, we need to pay attention to the protecting group of polymers, especially the protecting g...

Study of resist outgassing by F2 laser irradiation

F2 laser lithography at 157nm is the most promising candidate of post-ArF excimer laser lithography. A major concern, however, is the deterioration of 157nm optics due to contamination under F2 laser irradiation. An evaluation of outgassed products of 157nm resist and their effect on optical materials and is therefore indispensable for F2 laser lithography. Semiconductor Leading Edge Technologies Inc. (Selete) and Komatsu Ltd. designed and constructed a resist outgassing evaluation system in order to develop exposure tools and resists for 157nm lithography. The system determines the negative effects of outgassing resist contaminants on the transmittance of optical materials under F2 laser irradiation. The system has two units. One unit collects resist outgas and analyzes sampled gas in a gas chromatograph mass spectrometer (GC-MS). The other unit is a resist outgassing adhesion unit, which measures the transmittance change of optical materials due to contamination adhesion in real-time. Our analysis showed that most outgassed products were from the resist protecting groups and photo acid generators (PAG) including small hydrocarbons like isobutene, benzene derivatives and dimethoxymethane. After irradiating a 157nm lithography resist with a total dose of 30J/cm2 the transmittance of a calcium fluoride (CaF2) substrate decreased from initially 90% to 85%. This was due to adhesion contamination as x-ray photoelectron spectroscopy (XPS) analysis showed an organic contamination deposition of over 5nm thickness on the CaF2 substrate.

Development and evaluation of an F2 laser for immersion interference lithography at 157nm

A two-beam interference lithography system based on a line-selected F2 laser has been developed. Resist patterns with a 60nm line and space (L&S) resolution were produced by the interferometer by F2 immersion lithography. The F2 laser performance had been especially optimized for this application. The spectral emission at the 157.53nm line was less than 1% of the main line emission at 157.63nm. The main line had a deconvolved spectral bandwidth of 0.84 pm (full width at half maximum (FWHM)). The degree of horizontal linear polarization was above 0.73 and the visibility of spatial coherence was larger than 0.83 at a pinhole distance of 0.1mm.

Evaluation systems of F 2 laser lithography materials

In this paper, we present an evaluation system for F2 laser lithography masks and resists and we report preliminary test results. The evaluation system has two subsystems that are based on very accurate measurement technology. One subsystem is used for mask evaluation, the other subsystem for resist evaluation. The mask subsystem consists of two units. One unit evaluates real size 6025 binary masks placed horizontally as inside steppers. This unit measures three parameters: 1) the real time in-situ transmittance at 157nm during F2 laser irradiation, 2) the in-situ VUV transmittance using a VUV spectrophotometer and 3) the deformation of the pellicle. The precision of transmittance measurement at 157nm is +/-0.5%. The precision of the pellicle deformation measurement is +/-0.1μm. The second unit of the mask subsystem collects samples of the mask outgassing and analyzes them in a gas chromatograph mass spectrometer. The resist evaluation subsystem consists of three units. 1) One unit determines negative effects of outgassing resist contaminants on the transmittance of optical materials under F2 laser irradiation, 2) the second unit analyzes the outgassing from resists and 3) the third unit examines the effectiveness of exposure tool purge nozzles to reduce outgassing contamination.

Evaluation of outgassing from a fluorinated resist for 157-nm lithography

We have evaluated the outgassing products and the in-situ transmittance of a contaminated CaF2 substrate for monocyclic fluoropolymers with four protecting groups: methoxymethyl (MOM), tert-butoxycarbonyl (t-BOC), menthoxymethyl (MM), and 2-cyclohexylcyclohexyloxymethyl (CCOM). We have also evaluated the same type of fluoropolymer with seven kinds of photo-acid generators (PAGs) added to a base fluoropolymer solution. We found little correlation between the total amount of outgassing from the polymer and the decreasing rate of the CaF2 substrate transmittance caused by outgassing adhesion. Although the MOM protecting group generated the largest amount of outgassing products, the most substantial decrease in the transmittance was observed for the t-BOC protecting group. Also, the outgassing products due to use of a PAG did not greatly reduce the absorption coefficient of a CaF2 substrate regardless of the kind of PAG. Therefore, the absorption coefficient of the outgassing-contaminated CaF2 substrate appears to be more sensitive to the type of protecting group, especially the t-BOC protecting group including a t-butyl unit, rather than the type of fluoropolymer or PAG. We analyzed the substrate surface contaminant due to the t-butyl unit by x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and time-of-flight secondary ion mass spectrometry (TOF-SIMS), and found that increasing amounts of organic compounds, containing mainly C- and H-atoms, were adhered to and deposited on the substrate surface with an increasing irradiation dose. We speculate that the contaminants on a CaF2 surface with or without an anti-reflective coating were formed not only through mere physical adsorption, but also through certain chemical combinations. We conclude that in terms of material design of the fluoropolymer resist for 157-nm lithography, we need to pay attention to the protecting group of polymers, especially the t-BOC or t-butyl protecting group, which generates isobutene product during 157-nm irradiation.

In-situ measurement of VUV optical materials under F2 laser irradiation

The 157nm molecular fluorine laser is regarded as the next generation light source for semiconductor exposure technology in the vacuum ultraviolet (VUV) region. Research for high performance F2 laser optical materials is therefore indispensable. In this paper, we describe methods and results of evaluating optical materials used in the 157nm region. In order to evaluate F2 laser optical materials, we have developed in-situ system, which measures the real-time transmittance at 157nm during laser irradiation and the transmittance in the vacuum ultraviolet (VUV) region directly after laser irradiation to avoid airborne contamination. The system is purged with high purity nitrogen gas during irradiation to reduce laser light absorption and to keep contamination at a minimum. Due to F2 laser irradiation cleaning, the transmittance of uncoated calcium fluoride (CaF2) samples initially rapidly then gradually increased during 50 million pulses (Mpls). Thereafter the transmittance remained constant. In addition, durability test results of CaF2 substrates and coatings are also presented. Especially coating quality varied enormously between suppliers.

Development of environmental control technologies for 157-nm lithography at ASET

Purging and reduction of out-gassing are very important issues that need to be treated in order to realize F2 laser lithography system. Several methods of purging are tried and out-gases from metals, O-rings, lubricants, and an adhesive are analyzed. Metal surfaces mainly release oxygen and water independent of surface roughness, Ni plating, or elements. Other substances are not detected by API-MS or GC-MS. Since O-rings are indispensable to make gas-tight structures, several kinds of O-rings made of fluoro-compounds are tested. Black fluoro-rubber o-ring, O-ring F, is recommended from the view of organic out-gassing but Teflon-based fluoro-elastomer, O-ring A, is a good candidate in terms of the water out-gassing. Greases emit a large amount of out-gases even when the samples are not irradiated by 157 nm laser. As an adhesive, Adhesive A is recommended because of the fact that it does not release as much organic and inorganic compounds which may absorb 157 nm laser light. Finally preliminary demonstration using a model exposure system is performed to obtain purging time for several cases.

In-situ measurements of VUV optical materials for F2 laser

F2 laser lithography (wavelength:157 nm) is a candidate of post-ArF excimer laser lithography. In order to test the characteristics of vacuum ultraviolet (VUV) materials for F2 laser lithography, we developed an evaluation system consisting of a 1 kHz F2 laser, an in-situ real-time transmittance measurement unit and an in-situ VUV spectrophotometer. The precision of the real-time transmittance measurement is +/- 0.5%. The precision of the VUV spectrophotometer measurement is +/- 0.5% for scanned wavelengths (140 - 300 nm) and +/- 0.1% for a constant wavelength (at 157.6 nm). Due to F2 laser irradiation cleaning, the transmittance of uncoated calcium fluoride (CaF2) substrates and of F2 laser coatings at first rapidly and then gradually increased. Thereafter the transmittance remained constant. Results of the real-time transmittance and the VUV spectrophotometer measurement were almost identical. In addition, durability tests of CaF2 substrates and of F2 laser coatings were performed with a 4 kHz F2 laser for more than 10 billion pulses (Bpls). After the initial transmittance increase of CaF2 substrates, no change in transmittance was observed during more than 10 Bpls. In order to maintain the CaF2 substrate transmittance, silicon compounds have to be removed from the purge gas and from the irradiation chamber where optical materials are placed. F2 laser coating quality varied enormously between suppliers.

Present status of development of gas-purging and chemically-clean technologies at ASET

Purging and contamination are two important issues that need to be treated in order to realize F2 laser lithography system. For the purpose of developing gas-purging and chemically clean technologies, we designed and constructed an experimental set-up. It is used for the study of purging and out-gassing evaluation in order to obtain useful data for development of exposure system. Preliminary experiments showed that purging condition has a strong effect on the residual oxygen and water concentration in the final gas-replaced atmosphere. And we have found that the amount of out-gas depends on the surface finish method of the material used through analyses of impurity gas examination with or without laser irradiation.